When operating at its balance point, i.e. at rated horsepower at rated speed, a diesel engine will develop an amount of torque determined by such horsepower and speed. An increase in load will cause the engine speed to decrease. The governor system for the engine will typically function to move the fuel rack of the fuel pump in a fuel increasing direction in response to a decrease in engine speed so that the pump will supply more fuel to the engine (up to a maximum allowable amount). The increased amount of fuel causes the engine to generate more torque to meet the increased load.
Also typically, the slope of the torque curve (engine torque vs. engine speed) is set for the engine system by a torque spring which resists movement of the fuel rack in a fuel increasing direction as the engine speed reduces below rated speed.
A commonly used torque spring arrangement includes a cantilever-mounted spring blade and a full load stop surface parallel thereto. A spring-engaging member movable with the fuel rack actuation engages the spring when the engine is operating at its balance point and causes the spring to deflect relative to its fixed end as the rack actuator moves the rack in a fuel increasing direction in response to a decrease in engine speed. Continued movement of the rack actuator in a fuel increasing direction will cause greater and greater deflection of the torque spring until the torque spring comes into engagement with the full load stop surface. Such engagement prevents further movement of the fuel rack actuator in a fuel increasing direction and thus provides a maximum limit to the amount of fuel which the fuel pump can deliver to the engine.
The amount of rack movement required to move the torque spring from its undeflected position to its fully deflected (and positively stopped) position is normally referred to as ".DELTA. rack." Different engines and applications require different values of .DELTA. rack to produce a desired torque-rise curve.
Currently, spacer members are disposed between the fixed end of the torque spring and the full load stop surface so that the amount of rack is established by the thickness of the spacer members. There are, however, several problems with the use of such spacer members as a determinant for .DELTA. rack.
One problem is that, if a different torque-rise curve is desired, the torque spring assembly must be partially disassembled and a different spacer substituted to provide a different .DELTA. rack. Additionally, a supply of spacer members of different thicknesses must be maintained on hand to enable such different values of .DELTA. rack to be set.
Another problem that occurs results from thickness tolerance of spacer members. During engine assembly, a spacer member of correct nominal size will be installed. Frequently, however, it will be found, when the assembled engine is tested that there is a sufficient spacer thickness variation such that the engine will not meet the torque curve specification. In such case, the engine must again be partially disassembled so that the improperly sized spacer member can be replaced by one of correct thickness.
As a consequence, there is a need for a torque spring arrangement wherein .DELTA. rack can be adjusted and set for an engine independently of the spacer thickness.
Proper operation of a torque spring assembly requires that the member which moves with the rack actuator be just engaged with the torque spring, but without deflection thereof, when the engine is operating at the balance point. Current torque spring assemblies present a problem of determining the correctness of this adjustment.